1. Field of the Invention
The present invention relates to a surface analyzer that employs the technique of PELS (proton energy loss spectroscopy), in which a sample to be analyzed is irradiated by accelerated ion beams such as proton beams, and the beams scattered from the top few layers of the sample are decelerated by passage through a deceleration tube, and the energies of the decelerated scattered beams are measured to analyze physical properties of the surface of the sample.
2. Prior Art
A brief explanation of the basical principle of the PELS and a conventional surface analyzer employing PELS will be made hereinafter with reference to the accompanying drawings.
Suppose a proton of mass number m, moving with velocity U hits an atom of mass number M at rest at point O in FIG. 1. After collision, the proton glances off at velocity V along a path deflected from its original path by a scattering angle .theta. while the atom M recoils at velocity W in another direction at a scattering angle .PHI.. Unlike electrons, the proton is heavy enough to impart motion to atom M and loses energy.
Since momentum is conserved in both x- and y-directions, the following two equations are established: EQU mU=mVcos.theta.+MWcos.PHI. (1) EQU O=mVsin.theta.-MWcos.PHI. (2)
If the collision is completely elastic, kinetic energy is conserved so that: ##EQU1## Eliminating W from Eqs. (1) to (3), ##EQU2## The plus sign in Eq. (5) indicated scattering at angle .theta. and the minus sign indicates scattering in opposite direction. Since this direction is attained by subtracting .theta. from .pi., only the plus sign should be taken in Eq. (5). It should also be noted that in Eq. (5) .GAMMA. is defined as follows: ##EQU3##
If the kinetic energy of the proton before collision is written as E.sub.0, ##EQU4## After collision, the kinetic energy of the proton is reduced to Ea. If the coefficient of attenuation K is defined as: EQU Ea=KE.sub.0 ( 8)
the following equation is obtained: ##EQU5## Eq. (9) is the formula for the energy of a proton scattered at angle .theta.. This energy is dependent on .theta. but only to a small extent. In other words, Ea need not be measured for all values of .theta. and it suffices to measure Ea for a single value of .theta. after scattering of the proton. The atom with which the proton has collided can be specified by the parameter .GAMMA.. In Eq. (9), .theta. is not a variable but fixed.
A scattered proton at angle .theta. then enters an analyzer in which a voltage V.sub.0 is applied between two parallel plates. A proton entering the analyzer through a slit will travel along a parabolic path and fall into either one of the channels in a micro channel plate.
The distance, L, from the slit and the falling position of the proton is increased as the energy of the proton increases. In other words, the energy of a proton Ea can be determined by measuring L.
If the charge on a proton is written as ze, the proton in the space of the analyzer will receive a force zeV.sub.0 /h in a direction perpendicular to the plates which are spaced apart by h.
The velocity of the proton travelling through the analyzer is divided into two components, one being longitudinal and the other being transversal (i.e., perpendicular to the plates). The speed of the longitudinal component u is constant and that of the transversal component v varies. The proton passing through the slit will form a constant angle .PSI. with the surface of the nearer plate. The three parameters, u, v and .PSI., can be correlated by the following equations: ##EQU6## These kinetic equations are equivalent to those expressing the motion of an object thrown outwards under gravity.
From Eqs. (26) and (27), the distance L over which the proton travels in the analyzer in a direction parallel to the plates can be calculated as: ##EQU7## The highest resolution is attained when .PSI.=45.degree.. Eq. (28) shows that the distance L from the slit to the falling position of the proton is proportional to the energy of the proton Ea. Therefore, the distribution of Ea, or the energy of proton, can be determined based on measurement of L.
In FIG. 2 showing the general layout of a prior art surface analyzer that employs PELS, ion beams such as proton beams extracted from an ionization source 2 are accelerated by an accelerating tube 6 and are optionally converged by a converging unit 8. Subsequently, the beams are deflected by a magnet for mass separation. After deflection, the ion beams impinge on a sample (not shown) in a scattering chamber 14. In order to irradiate a selected area of the sample by the ion beams, a slit (not shown) with a diameter of about 1 mm.PHI. is provided on the beam line preceding the entrance of ion beams into the scattering chamber 14. If He and other elements are used as sources of ion beams, not only monovalent but also divalent ions may be scattered from the sample, thereby increasing the complexity of measurement, so in the actual operation protons that exist only in the form of monovalent are employed ions. In FIG. 2, the numeral 12 denotes a vacuum pump.
The ion beams impinging on the sample are scattered by the top few layers of the sample. The scattered beams generally have varying values of energy and in order to reduce this spread of energy, a slit (not shown) of about 2 mm.PHI. is provided on the transport line of the scattered beams. The scattered beams passing through this slit are decelerated by a decelerating tube 18 and their energy spectrum is analyzed with a measuring instrument 20.
The method for determining the potential of the decelerated ion beams is hereunder described by referring to both FIGS. 2 and 3. Suppose that the ion beams are extracted from the ionization source 2 at a voltage of Ve, that the extracted ion beams are accelerated in the accelerating tube 6 by a voltage of V, and that the scattering chamber 14 has a zero potential assuming that it is grounded. Then, the total acceleration voltage of the ion beams, Va, is equal to the sum of V and Ve. If a table 19 (insulated from the ground) for mounting the measuring instrument 20 has a potential of Vd that is equal to Va minus an offset voltage Vo (that is, if the deceleration voltage Vd of the decelerating tube 18 is equal to Va-Vo), the scattered beams that enter the measuring instrument 20 after deceleration have an energy of q.times.Vo (eV) (disregarding the energy loss that occurs when the beam hit the sample) where q is the unit charge on an ion, for example, a proton.
The accelerating energy of the ion beams by which the sample is to be irradiated is preferably high for various reasons among which the principal one is that highly energized ion beams have a low probability of neutralization. The ion beams typically have an accelerating energy of about 100 keV. On the other hand, the scattered beams preferably have a low energy for the purpose of achieving high-precision measurement of their energy spectra. In a typical case, the scattered ions are decelerated to less than about 1 keV.
Based on the measurement of the energy spectra of the thus decelerated scattered beams; the crystalline structure and other physical properties of the surface of a solid sample can be investigated. By referring to FIG. 4, if the ion beam 3 loses an energy of .DELTA.E because of collision with the sample 15 and if the scattered beam 4 entering the measuring instrument 20 has an energy of E, then the following relationship holds good: EQU .DELTA.E=qVo-E (10)
because the energy E of the scattered beam 4 entering the measuring instrument 20 is expressed as: EQU E=qVa-.DELTA.E-q(Va-Vo) (11)
which can be rewritten as Eq. (10).
The measuring instrument 20 shown in FIG. 4 consists of an energy analyzer 21 and a detector 22 such as a channeltron. Writing V.sub.ESA for the voltage applied to the energy analyzer 21, the energy E can also be expressed as follows: EQU E=kqV.sub.ESA ( 12)
where k is a constant.
As can be seen from Eqs. (10) and (12), the spectrum of energy loss can be determined by varying either the offset voltage Vo or the voltage V.sub.ESA. Consider here the beams that are scattered by atoms in the top three monolayers of a sample; the beams scattered in the second and third top layers travel a longer distance in lattices than the beams scattered in the topmost layer and hence undergo more energy loss .DELTA.E, yielding a spectrum of the shape shown in FIG. 5.
In the conventional PELS equipment, the scattering angle .theta. (see FIG. 4) can only be set to a small angle, for example, below 10.degree. for the following two reasons: as the scattering angle is increased from 0.degree., the solid angle .theta. of a scattered beam 4 is decreased to lower the efficiency of beam detection; secondly, if the scattering angle .theta. is too large, individual parts of the equipment will mechanically interfere with one another. However, if the scattering angle .theta. is below 10.degree., low scattering approach has the disadvantage that it is highly susceptible to the surface state of a sample to be analyzed and undergo "energy straggling" due to double scattering caused by the asperities on the sample, and energy spectra of the scattered beams become too broad to guarantee a very high precision of analysis.
If one wants to measure the origin of energy loss (i.e., the point at which a scattered beam 4 of .DELTA.E=0 enters the measuring instrument 20) or the energy resolving power of the equipment by admitting the ion beams 3 directly into the instrument 20 without allowing them to be scattered by the sample 15, it is necessary to modify the connection of the beam transport line and other parts of the equipment subsequent to the scattering chamber 14 in such a manner that the scattering angle .theta. will be 0.degree.. This reconnecting operation and subsequent jobs for achieving alignment are highly time-consuming. In addition, the reassembled equipment may not guarantee the same level of precision as achieved before.
Furthermore, Eq. (9) shows the in the neighborhood of .theta..congruent.0, k does not vary with .GAMMA., so that one value of .GAMMA. can not be easily distinguished from another.